Plasma coated antenna



United States Patent 3,544,998 PLASMA COATED ANTENNA Paul E. Vaudenplas, 80 Avenue de la Brabanconne, Brussels, Belgium, and Andr M. Messiaen, 19 Avenue Colonel Picquart, Schaerbeek, Belgium Filed Dec. 8, 1967, Ser. No. 689,151 Claims priority, applicatim Belgium, Dec. 19, 1966,

69 ,389 Int. C1. H01q 1/26, 1/28, 9/00 U.S. Cl. 343-701 8 Claims ABSTRACT F THE DISCLOSURE FIELD OF THE INVENTION This invention relates to antennas and more particularly to a novel plasma coated antenna wherein the thickness of the plasma sheath is varied to produce a desired resonant frequency.

DESCRIPTION OF THE PRIOR ART In the past, various theories have been expounded for predicting the behavior of an antenna surrounded by a layer of plasma. However, these theories have neglected the experimental fact that there exists an ionic sheath which separates the antenna from the plasma environment. Prior antennas for a given wavelength and power output have required congurations of a certain size, for example, in aircraft and spacecrafts, that limited the overall operation of the vehicle utilizing the antenna. Furthermore, cumbersome elements exist on vintage antennas which limit the ability to rapidly tune the antenna.

Accordingly, it is an object of the present invention to provide a novel plasma coated antenna wherein the thickness of the sheath between the outer surface of the antenna and the plasma layer is varied to tune the antenna.

Itis a further object of the present invention to -provide a novel plasma coated antenna wherein the density of the plasma may be controlled to tune the antenna,

It is a still further object of the present invention to provide a novel plasma coated antenna which is relatively small compared to the wavelength in free-space and provides considerable radiation capabilities.

It is another object of the present invention to provide a novel plasma coated antenna which may be rapidly tuned.

It is yet another object of the present invention to provide several novel plasma coated antennas cooperating to provide additional ydetection capabilities.

SUMMARY OF THE INVENTION In accordance with the objects set forth above, the present invention provides a novel plasma coated antenna enclosed within a low-pressure housing. A plasma environment exists within the housing that exhibits a sheath located between -the plasma and the outer extremities of the antenna. The antenna may be selectively tuned by varying either the thickness of the sheath or the density of the plasma. Several antennas may -be adapted to cooperate with each other to provide for the detection of operating frequencies of other radiation devices.

BRIEF DESCRIPTION `OF THE DRAWINGS Additional objects, advantages, and characteristic features of the present invention will become readily apparen-t from the following detailed description of preferred embodiments of the invention when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a plasma coated antenna system shown partially sectioned in accordance with the present invention;

FIG. 2 is a theoretical curve calculated from the parameters exhibited by the plasma coated antenna system of FIG. 1;

FIG. 3 is an experimental curve of the results obtained in the study of a plasma coated antenna system shown in FIG. l; and

FIG. 4 is a functional block diagram of a typical aireraf-t having several plasma coated antennas therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Refer-ring now to FIG. 1, there is shown one of the em- Ibodiments of a plasma coated antenna system 10 constructed in accordance with the principles of this invention in which a spherical antenna 11 is enclosed within a discharge tube 12. The spherical antenna 11 comprises two isola-ted hemispheres 11a and 11b, respectively, which may be made of any suitable metallic material. The discharge tube 12 may be a low-pressu-re mercury-discharge tube constructed of any suitable material, such as a ceramic material, for example, glass or liber glass.

A plas-ma 13 is contained within the discharge tube 12. A cathode 14 and anode 15 are located in their respective lateral tubes 16 and 17 of the discharge tube 12. The lateral tubes 16 and 17 may be covered by two high frequency absorbers 18 and 19, respectively, in order to minimize spurious effects. The cathode 14 and the anode 15 are connected to a source 26, via lines 26a and 2611, which may be utilized to control the density of the plasma 13. The density of the plasma 13 may be Varied by adjusting the cur-rent of the discharge Where the density of the plasma 13 is approximately proportional to the current density. .When the density of the plasma 13 is changed, the frequency at which the antenna 20 will resonate will change. Thus, the antenna 11 may be tuned by varying the density ofthe plasma 13.

By experiment, the inventors have discovered the existence of a plasma sheath 23, between the outer surface of the antenna 11 and the plasma 13, in which the electron density is very low. This plasma sheath 23 consists almost entirely of positively charged ions, and acts electrically like a vacuum, thus, isolating the antenna 11 yfrom the plasma 13 and is capable of alterning the electrical characteristics of the antenna 11. A bias source 20, which may be a typical DC source, is shown connected to the two hemispheres 11a and 11b by the lines 20a and 20b, respectively. The bi-as source 20 may be utilized to vary the voltage potential of the antenna 20 and consequently the lradial thickness of the sheath 23.

While the plasma 13 has an electron-plasma density, ne, the plasma sheath 23 has an electron-plasma density nearly equal to zero (178:0). The plasma sheath 23 can be physically observed as less luminous than the plasma 13. The plasma sheath 23 is shown as having a thickness (b-a), Where (a) is the radius of the antenna 11 and (b) is the distance from the center of the antenna 11 to the outer limits of the plasma sheath 23. The distances from the center of the antenna 11 to the inner rim and outer rim of the discharge tube 12 are shown as (c) and (d), respectively.

When operated in the condition useful to this invention, the plasma 13 has an inductive reactance as opposed to the capacitive reactance of the plasma sheath 23 and the area exterior to the discharge tube 12. There is a resonance phenomenon between the plasma 13 and the area exterior to the discharge tube 12 and a corresponding resonance phenomenon between the plasma 13 and the plasma sheath 23. The resonance phenomenon which results from the interaction of the plasma and the area exterior to the discharge tube is insensitive to the thickness of the plasma sheath 23. n the other hand, the resonance phenomenon experienced between the plasma 13 and the plasma sheath 23 is strongly influenced by the thickness of the plasma sheath 23. The thickness of the plasma sheath 23 is determined by the direct voltage applied to the antenna from the bias source 20. When this voltage is varied, the thickness of the plasma sheath 23 is changed, thus, the resonance is changed. Therefore, by changing the thickness of the plasma sheath 23, the antenna 11 may be tuned. Thus, in the operation of the plasma coated antenna system 10, the antenna 20 may be selectively tuned by either changing the thickness of the plasma sheath 23 or by varying the density of the plasma 13. In addition to the selective tuning characteristics of the plasma coated antenna system 10, the antenna 20` may be rapidly tuned by the two aforementioned operations.

An ultra-high frequency generator 28 may be utilized to excite the antenna 11. A coaxial cable 21 is shown connecting the ultra-high frequency generator 28 to the antenna 11. A typical Balsun 22 may be placed around the coaxial cable 21 to match the generator 28 to the antenna 11.

Theoretically, the plasma is assumed uniform and collision-free and described in the cold-plasma limit by its equivalent permittivity epgeoU-weZ/), where w is the H.F. angular frequency and we the electron-plasma frequency. The plasma sheath or we has a vacuum permittivity eo. Assuming that the high frequency potential difference applied between the two hemispheres 11a and 11b has an amplitude ZVO, the Helmholtz equation may be solved in the different media, that is the plasma sheath 23, the plasma 13, the wall of the discharge tube 12, and the outer vacuum. As stated earlier the distance (b-a) is the thickness of the plasma sheath 23. In addition, the distance (c-b) is the thickness of the plasma 13, and the distance (dc) is the thickness of the wall of the discharge tube 12. Because of the symmetry of the problem, the solutions are expressed in odd Legendre polynomials Pn(cos 6); n=1, 3, 5 The boundary conditions are the continuity of the tangential components of the electric field E and the magnetic field H.

Upon solving this set of equations, one finally obtains |E| in the vacuum outside as a function of t2=we2/w2. The [Eel was computed for 0=90', i.e. in the equatorial plane at r== cm. (this is where a detecting antenna was placed in the experiments, in order to minimize the effects of spurious reflections on the walls of the laboratory). Since- E, has the usual km0) ,(kor) behavior of a spherical radiated field, it is immaterial at what distance r the amplitude of the field is computed.

Referring now to FIG. 2, there is shown a theoretical curve 30 of the radiated [E61 at 0=90 as a function of t22=we2/w2 (Ea in v./cm. with V=1 v.). The different n=l, n=3 etc. peaks are those resonances resulting from the appropriate Pn(cos 0), and are due to a resonance phenomenon between the plasma 13 and the outer vacuum; these resonances are very insensitive to the thickness of the plasma sheath 23. The different n=1, 11:3 etc. peaks are the corresponding resonances due to a resonance phenomenon between the plasma 13 and the plasma sheath 23. These latter resonances depend, as stated earlier, very strongly on the plasma sheath thickness, and can become much more important than the other ones if appropriate values of a' b and c are chosen. It should be noted that the field which is radiated by the antenna in the absence of plasma (522:0) is very small, since the radius of the antenna is much smaller than the vacuum wavelength ko (a )\o). It should also be carefully noted that an antiresonance (E=0) occurs for w=we. When the collisions are small, i.e. v/ w 1 (v1-collision frequency),

this results in a blackout at the plasma frequency, which corresponds to the well known blackout phenomenon observed with satellites. Thus, it is theoretically possible to tune the antenna to avoid the adverse effects of the aforementioned blackout phenomenon.

Referring now to FIG. 3, these is shown a corresponding experimental curve 35 with [Eel as a function of IdiSa N aQ2=we2/w2 N =equilibrium plasma electron density). The experiments were carried out at a fixed operating frequency of w/21r=30() mHZ. The Idis is thus proportional to Q2, since the experiments were carried out at a given operating frequency w/21r=300 mHz. With the direct bias potential used, the sheath thickness is b-azlSrD (rD=Debye length). As predicted theoretically, the main resonance to the right (n=l) varies in both position and magnitude as a function of sheath thickness, while the n=l main resonance to the left is insensitive to this sheath thickness. The antiresonance is ob- .served at w=we, and the value of we has been checkedby an independent method. Since V/we() (in fact v/we-lO-), one does not observe a zero of E0 at antiresonance but a nonzero minimum value. The experiments were done at a rather low power (20:1 w.).

As shown in FIG. 3, the field radiated by the antenna 11 in the absence of plasma 13 (522:0) is very small when compared to the n=1 resonance field due to the plasma 13 and its sheath 23. The radiated field exhibited was 20` times stronger than the field in the absence of plasma; therefore, the radiated power was 400 times greater than without plasma. This radiation factor results from the fact that the reactance is practically zero and the impedance is thus resistive (in this particular experiment the radiation resistance was very small and was approximately equal to 0.39). For resonance and for a given voltage of the ultra-high frequency generator 28, the A experimentally observed radiated power was limited by the internal impedance of the generator. In conclusion, it should be stressed that the resonant behavior of an antenna surrounded by plasma exhibiting a resultant plasma sheath enables the antenna to be tuned, even when its dimensions are smaller than the vacuum Wavelength.

Referring now to FIG. 4, there is shown a typical aircraft 40, including plasma coated antennas 41 through 44. While it is understood that the plasma coated antennas 41 through 44 could be placed in numerous locations throughout the aircraft 40, the antennas 41 through 44 are shown in their respective locations for sake of clarity. The plasma coated antennas 41 and 42 may be utilized for transmitting information to and receiving information from targets located forward of the aircraft 40, while the plasma coated antennas 43 and 44 may be utilized for transmitting information to and receiving information from targets off to the side of the aircraft 40.

The plasma coated antennas 41 through 44 are shown in their different configurations from the plasma coated antenna 11 of FIG. 1. It should be understood that the antennas 41 through 44 are constructed to adapt themselves to the aircraft 40. Furthermore, while the first ernbodiment was described with particular reference to antennas having a spherical configuration, it should be understood that the practice of this invention is not necessarily limited thereto, but may be practiced to equal advanage utilizing other configurations, such as, rectangular configurations. It can be appreciated that the plasma coated antenna system 10 was shown in a spherical configuration in order that the mathematical equations would be of the type that would faciliate solving.

The anennas 41 and 42, and 43 and 44, respectively, may be utilized to provide the detection of the operating frequency of targets, for example, jamming and counterjamming systems. One of the capabilities of a plasma coated antenna, as described earlier in this invention is the ability to rapidly tune the antenna. This capability is very significant when one is involved with jamming and counter-jamming techniques,

Furthermore, while FIG. 4 shows a typical aircraft 40, it should be understood that the practice of this invention is not necessarily limited thereto, but may be practiced t equal advantage utilizing other air vehicles, such as, spacecraft. A spacecraft may utilize the plasma coated antenna of the type shown in either FIG. l or FIG. 4 for transmitting and receiving purposes. In addition, a plasma coated antenna may be utilized in a spacecraft to overcome problems encountered during re-entry into the earths atmosphere, for example, problems of radio blackout.

Thus, although the present invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modiiications obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope, and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. An antenna system comprising:

first means for radiating energy;

second means surrounding said first means for providing a low pressure housing, said second means having a plasma condition provided therein adapted to exhibit an expandable plasma sheath adjacent to the outer edge of said iirst means; and

third means including a iirst source adapted to change the bias voltage applied to said first means and a second source adapted to control the discharge current of said second means. v

2. An antenna system as recited in claim 1 wherein said second means is a low-pressure mercury-discharge tube.

3. An antenna system as recited in claim 1 wherein said first means comprises a metallic antenna.

4. An antenna system as recited in claim 3 wherein said antenna comprises two members located in proximity to each other.

5. An antenna system as recited in claim 3 wherein said metallic antenna comprises two hemispheres located in proximity to each other.

6. An antenna system as recited in claim 3 wherein said iirst means includes a high frequency generator for exciting said metallic antenna.

7. An antenna system as recited in claim 1 wherein said third means comprises a bias voltage source adapted to vary the thickness of said plasma sheath and a voltage source with an anode and cathode located within said second means for changing the density of said plasma.

8. An antenna system proviling detection capabilities, operating in an aircraft, comprising:

at least one pair of plasma coated antennas;

iirst means respectively housing each said pair of plasma coated antennas, said first means having plasma conditions provided therein attached to exhibit expandable plasma sheaths respectively adjacent to the outer edges of said plasma coated antennas; and

second means respectively cooperating with said plasma coated antennas and said iirst means for selectively tuning said plasma coated antennas.

References Cited UNITED STATES PATENTS 2,525,624 10/1950 Stahl et al. 343-701 X 2,615,126 10/1952 Kennebeck 343-701 2,641,702 6/ 1953 Cohen et al. 343-701 2,703,363 3/1955 Rines et al. 343-701 X 2,968,037 1/1961 Thompson 343-701 3,080,523 3/1963 Miller 343-701 X ELI LIEBERMAN, Primary Examiner T. VEZEAU, Assistant Examiner U.S. Cl. X.R. 343-705, 745

*'0-050 UNITED STATES PATENT OFFICE (fl/69) V CERTIFICATE OI*` CORRECTION Patent No. 3,544,998 f Dated December 1, 1970 Inventor(s) 'Paul E. Vandenplas and Andre! M. Messiaen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I .Column 3, line 25, "Balsun" should. be -Ba1un Column 3, line 58, equation "(Ea in v./cm. with Vo l v.)

Column 4, line 6, "these" should be "there-- Column 4, lines 30, 31 "This radiation factor results from the fact that the reactance is practically zero" should be --This radiation factor results from the fact that the reactance presented by the plasma antenna system 10 when in resonance is practically zero-- Smm mi) SEALED IIARZ 1971 (SEAL) Auen:

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